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Old 08-06-2006, 02:40 PM   #1
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Yikes! No Idle Adjustment - Fast Idle.

I am always AMAZED at the breadth of knowledge of the readers on this site.

So, here is a Tow Vehicle question for you.

I just purchased a 94 GMC Suburban with a 454 engine and TBI injection. I am now learning that it idles a bit too fast. When I stop at a traffic light, I have to hold pressure on the brakes to keep from going forward. If I release the brakes, it will creep forward pretty fast. I have looked everywhere for an idle adjustment and there is NONE! And...it hasn't escaped my attention that the fast idle burns a lot of GAS! I purchased a Haynes Repair Manual, but for idle adjustment, it simply says "take it to the dealer!" A lot of help that is!

Any suggestions on what to check or what to do would be appreciated. Bill
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Old 08-06-2006, 02:59 PM   #2
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Does the check engine light come on while it is idling? You may have an IAC that is blocked partly open.
The IAC, or Idle Air Control motor, is located on the right rear of the throttle body assembly. It should take about a 15/16" wrench to lossen it, and it unsrews easily. Clean out the hole (inside the throttle body) and the V shaped plunger on the motor. Install it, and see if the idle goes down.
If the check engine light is on, you can get clues of what is going on from them, although what the code says, is not always what the problem is.
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Old 08-06-2006, 04:35 PM   #3
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Quote:
Originally Posted by overlander63
Does the check engine light come on while it is idling? You may have an IAC that is blocked partly open.
The IAC, or Idle Air Control motor, is located on the right rear of the throttle body assembly. It should take about a 15/16" wrench to lossen it, and it unsrews easily. Clean out the hole (inside the throttle body) and the V shaped plunger on the motor. Install it, and see if the idle goes down.
If the check engine light is on, you can get clues of what is going on from them, although what the code says, is not always what the problem is.
No, the "check engine" light is not on. I will remove the IAC and clean it and re-install it to see if that helps. I'll let you know. Thanks, Bill
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Old 08-06-2006, 05:14 PM   #4
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Something else it could be, is simply a broken vacuum line. It will also make the engine rev up a bit, so if the cleaning (which won't hurt anything) doesn't make a difference, check for cracked or broken vacuum lines.
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Old 08-06-2006, 06:41 PM   #5
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My son's '91 C1500 started idling faster and we found that it was a brake booster going bad. This goes right along with the vacuum leak suggestion.
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Old 08-06-2006, 07:07 PM   #6
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Fast Idle

Spray WD40 around the base of the TBI where the TBI meets the hot water pod and also where the hot water pod meets the intake manafold. This a common problm with the TBI on the 454 engines. If the gaskets are bad and leaking vacuum the engine idle will slow down when the WD40 enters the hole in the gasket. If this is the case then you will need to replace the gaskets. Also check the same way where the manafold gaskets meet the heads. You do have a vacuum leak.


Good luck,
Let me know the results
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Old 08-06-2006, 07:15 PM   #7
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Also disconnect the rubber hose going to the brake booster. Put your finger over the hose and if the idle slows down to normal then it is the diaphram in the brake booster.
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Old 08-06-2006, 08:22 PM   #8
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check the base gasket on the tbi unit.

a quick check is to take an unlit propane torch (gas on) and hold it around the base of the tbi unit.

if it revs up you have found the problem.

john
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Old 08-06-2006, 11:20 PM   #9
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Quote:
Originally Posted by Pat370
Spray WD40 around the base of the TBI where the TBI meets the hot water pod and also where the hot water pod meets the intake manafold. This a common problm with the TBI on the 454 engines. If the gaskets are bad and leaking vacuum the engine idle will slow down when the WD40 enters the hole in the gasket. If this is the case then you will need to replace the gaskets. Also check the same way where the manafold gaskets meet the heads. You do have a vacuum leak.


Good luck,
Let me know the results
Pat370
Throttle body base gaskets are a common problem, above is a simple check for that----Pieman
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Old 08-07-2006, 12:16 PM   #10
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Fast Idle

Thanks for the ideas - Craig, Pat370 and John. I will try the WD40 next. I just got a new can of that. Unfortunately, I sold my propane torch in a garage sale a few years ago, thinking that I would never need it again! Now I wish I had it!

Terry- I removed and cleaned the IAC valve (pintle) and the valve seat and re-installed it and that did not fix the problem. However, I have some questions about what is normal...or NOT about this valve. I expected the pintle to freely move in and out, since it has a spring for tension.......but it does not! It takes a great deal of pressure to make it move in or out. And when it does move, it moves very slowly and feels like it meets a great amount of friction. Then it occured to me that this was a MOTOR and the pintle rod, through some kind of a gear reduction mechanism was connected to this motor and when you tried to push the pintle in and out you had to overcome the gear reduction to turn the motor. If that's true, then maybe this great resistance to movement was normal? Can you clarify that? If this is not normal, then I've got a bad IAC valve. I also measured the resistance between the connection pins A-B and C-D and got 54 ohms on each pair. The Haynes Manual says that is normal.

I'll wait to hear something before I try the WD-40. Bill

Oh, I forgot to mention that at idle, the RPMs on my Tach is 1100 RPMs with the Trans in neutral and 1000 with the Trans in Drive. Don't know how accurate the Tach is.
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Old 08-07-2006, 12:26 PM   #11
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give it a shot of WD can't hurt!

i had the same exact problem on a late 80's chevy. when i pulled the tbi body off there was literally nothing left of the gasket! huge air leak!

john
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Old 08-07-2006, 12:46 PM   #12
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Fast idle

When the IAC valve is bad the engine will die when your foot is removed from the gas pedel. I am sure that you have a bad TBI base gasket.

Let me know how you come out,

Pat370
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Old 08-07-2006, 05:24 PM   #13
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If the gasket is bad enough you can look down through the throat of the throttle body and see that it's out of place---pieman
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Old 08-07-2006, 05:43 PM   #14
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Quote:
Originally Posted by Pat370
When the IAC valve is bad the engine will die when your foot is removed from the gas pedel. I am sure that you have a bad TBI base gasket.

Let me know how you come out,

Pat370
Exactly. If it was gunked up, it could not close enough to control idle, now that it is clean, he can check for a vacuum leak, and be fairly certain it is not (almost) the only electronic control part that is causing the problem.
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